The cytokine interleukin (IL)-15 is crucial for NK cell development, survival and proliferation. The receptor for IL-15 (IL-15R) consists of three subunits: the alpha chain, unique to IL-15R; the beta chain, shared with IL-2R; and the common gamma (gamma-c) chain, shared with the receptors for IL-2, IL-4, IL-7, IL-9, and IL-21. IL-15Ralpha and IL-15 are co-expressed in the same cells and exported to the plasma membrane as a complex, with IL-15Ralpha acting as a chaperone for IL-15. Therefore, IL-15 is usually presented in trans by cells that express the IL-15RalphaIL-15 complex, such as stromal cells and dendritic cells (DC), to cells that express the beta-gamma-c chains, such as T cells and NK cells. Signal transduction from the beta-gamma-c chains activates two pathways: one that leads to a transcriptional response and cell survival through JAK1/3-dependent phosphorylation of Stat5, and the other, which activates cell proliferation through PI-3K-Akt-mTOR dependent phosphorylation of the ribosomal protein S6 by the p70-S6 kinase. We found that IL-15Ralpha expressed by DC is internalized by NK cells upon trans-presentation and that the IL-15RalphaIL-15 receptor colocalized with IL-2Rbeta chain in intracellular NK cell compartments. Inhibition of matrix metalloproteases (MMP) to prevent shedding of IL-15RalphaIL-15 did not reduce trans-endocytosis of intact IL-15Ralpha and activation of ribosomal protein S6 kinase. In contrast, MMP inhibition blocked the release of the extracellular domain of IL-15Ralpha and reduced the phosphorylation of Stat5. An IL-15RalphaIL-15 complex coupled covalently to large beads was a better activator of Stat5 rather than S6 phosphorylation. Silencing of the small GTPase TC21, which promotes trans-endocytosis of MHC into T cells, inhibited S6 but not Stat5 phosphorylation in NK cells. These results demonstrate that signals for survival of NK cells and signals leading to proliferation can be selectively regulated according to the form of IL-15Ralpha and to its localization in different NK cell compartments. Trans-endocytosis of membrane-associated IL-15RalphaIL-15 provides a mechanism for NK cell expansion that is not afforded to soluble IL-2, and has profound implications for the stimulation and use of NK cells in cancer immunotherapy. NK cells are an important component of the immune system, contributing to both pathogen defense and tumor immune surveillance. Clinical data has shown a correlation of higher NK cell infiltration into tumors with better immunotherapy outcome. Tumor tissues may express ligands for NK activating receptors, such as ULBP and CD155, which are ligands for activating receptors NKG2D and DNAM-1, respectively. Unlike T and B cells, which use a large repertoire of receptors to achieve antigen specificity, NK cell activity is regulated by a limited number of germline-encoded activating and inhibitory receptors. Activation of resting NK cells requires synergy of specific pairs of activating receptors. The CD28B7 family of receptorligand pairs regulates T cell responses through costimulation and coinhibition. It includes checkpoint inhibitors, such as PD-1, which limit anti-tumor responses. CD28 homologue (CD28H, encoded by the TMIGD2 gene) has been reported as a costimulator and coinhibitor of T cells. Here we show that CD28H is a coactivation receptor on NK cells that synergizes with receptors 2B4 and NKp46, but not NKG2D and DNAM-1, to induce NK cell degranulation and lysis of target cells. We found that CD28H is expressed on resting, unstimulated human NK cells, and that its expression was lost after sustained stimulation in IL-2. To dissect signal transduction through CD28H, we generated a panel of mutants to disrupt the three potential tyrosine phosphorylation sites within the intracellular tail of CD28H. Mutation of Y192 alone abolish CD28H phosphorylation and NK activation through CD28H. CD28H binding to its ligand B7-homologue 7 (B7H7, encoded by the HHLA2 gene) enhanced antibody-dependent cellular cytotoxicity of NK cells. NK-dependent lysis of a B7H7+ tumor cell line was inhibited by the dominant inhibitory receptor for HLA-E. However, expression of a chimeric antigen receptor, consisting of CD28H fused to TCRzeta, in NK cells overcame inhibition and triggered tumor cell lysis. Thus, CD28H is a new addition to the arsenal of activation receptors used by NK cells to detect tumor cells. Natural killer (NK) cells are innate immune effector cells regulated by many germline encoded activating and inhibitory receptors. The DAP12-associated activating receptor KIR2DS4 has been linked with multiple diseases including cancer, disorders of pregnancy, and resistance to HIV. However, the ligand(s) for KIR2DS4 have remained poorly defined, and the role of this receptor in immune responses is unclear. We have shown that human KIR2DS4 is a highly peptide-specific receptor for the human MHC-I molecule HLA-C*05:01. Of over 60 different peptides tested, only two conferred binding to KIR2DS4. Recognition of these peptides presented by HLA-C*05:01 potently activated KIR2DS4+ NK cells to degranulate and produce IFN-gamma and TNF-alpha. Recombinant peptide:HLA-C alone was sufficient to activate KIR2DS4+ NK cells and we estimated the number of HLA-C molecules required for stimulation to be in the range of only a thousand per cell. An alignment search of the KIR2DS4 binding peptides identified an epitope in recombinase A (RecA), a highly conserved bacterial protein. RecA epitopes from the pathogens Chlamydia, Campylobacter, Brucella and Helicobacter were presented by HLA-C*05:01, bound to KIR2DS4, and activated KIR2DS4+ NK cells. By sequence alignment we predict that hundreds of species of bacteria contain RecA epitopes that can be presented by HLA-C*05:01 and bind KIR2DS4. These data provide clear evidence that KIR2DS4 is a highly peptide specific activating receptor and suggest that KIR2DS4 evolved to play a role in immune defense to bacteria.